The conversion of carbohydrates into polyhydric alcohols (sugar alcohols) under hydrogenation conditions and in the presence of ruthenium on solid carrier materials is known. For example, U.S. Pat. No. 2,868,847, describes a process for the catalytic hydrogenation of carbohydrates in which ruthenium on inert carrier materials such as carbon, aluminum oxide, silicon dioxide, kieselguhr, silica gel, or diatomaceous earth, is used as catalyst. The carbohydrates used include monosaccharides such as dextrose and fructose and disaccharides such as sucrose and lactose. Dextrose was hydrogenated to sorbitol, and sucrose and lactose were hydrolyzed and hydrogenated to hexitols.
Moreover, the use of ruthenium on animal charcoal as catalyst in the hydrogenation of monosaccharides is described by N. A. Vasyunina et al. in Izv. Akad. Nauk. SSR Khimicheskaya, 4 (1969), p. 848-854. Methods for the catalytic hydrogenation of carbohydrates in the presence of ruthenium on crystalline aluminosilicate clay or of ruthenium-zeolite catalysts are known from German published application (DE-OS) No. 2,555,856 and the patent literature cited therein.
It has previously been proposed to continuously carry out the catalytic hydrogenation of carbohydrates in the presence of suspended finely divided ruthenium carrier catalysts (see, U.S. Pat. No. 2,868,847 and German published application (DE-OS) No. 2,555,856). However, this procedure is characterized by a number of serious disadvantages. First, employing the catalyst in suspension hydrogenation in finely divided form makes it necessary to remove the catalyst from the resulting reaction mixture by complicated filtration systems. Second, quantitative elimination of the catalyst is necessary, not only just to purify the hydrogenation products. When precious metal catalysts are used, as in the present case, their virtually quantitative recovery is also a prerequisite for the profitability of the process. Besides, it has been found that the ruthenium carrier catalysts separated after the hydrogenation show a reduced activity when re-used, so that the ruthenium carrier catalysts must be regenerated after the third pass at the latest. If the separated catalysts has to be regenerated or processed for recovery of ruthenium, it must be freed from the adhering hydrogenation product by intensive washing. Here, too, it is disadvantageous that the catalyst is present as a fine powder. Third, in continuous hydrogenation with suspended catalysts there is a danger that sedimentation will lead to clogging in the reactors and other parts of the hydrogenation installation. And lastly, the abrasiveness of the catalyst carrier material causes increased wear of the installation parts coming in contact with the suspension.